Ferry Vessel Propeller Wash Measurements

Abstract

We investigated the hydrodynamic causes of severe erosion at the Washington State Ferries ferry terminal in Kingston, WA, where a cliff-like bathymetric feature has shifted shoreward in recent years, forcing repairs on the slip’s bridge pilings. High resolution measurements of velocity and estimates of turbulent bed stress were made during vessel arrivals and departures during two deployment periods in March and April 2018. Calculated bed stresses (and maximum velocities) were found to be 10-100 (30-50) times larger during vessel arrivals and departures than background levels. The structure of the wash is modified by the bathymetry such that steeper bathymetry leads to more reflection of the propeller wash. Bed stresses were higher during vessel departures than arrivals, corresponding to higher vessel acceleration during departures. Additionally, during departures, lower tidal stages correspond to higher bed stresses, because the propellers are closer to the seabed. During arrivals, larger vessels generated higher bed stresses. In this work, we develop, assess, and use an empirical model for bed stress that takes into account these dependencies. Using this model, we determine that the most important factor for cumulative vessel stress is the frequency of use of a given slip. Additionally, shallower ground lines will be exposed to higher bed stress than deeper ground lines, so modifications to the existing bathymetry should take into account both the effect of shallower depth and milder slope.